The subject matter described herein relates to methods and systems for forming metal alloy features on microfeature workpieces and controlling composition gradients within the deposited metal alloy features.
Metal alloys are utilized in numerous applications in the microelectronic industry. For instance, Permalloy and other magnetic alloys (which are based on nickel, cobalt and iron) are used in giant magnoresistive heads and magnoresistive heads. Metal alloys are also used as conductive features for interconnects and vias. Noble metal alloys are used as electrodes for capacitors. Other metal alloys, such as lead-tin alloys and lead-free alloys are used as solders for mounting microelectronic devices to substrates.
When metal ions in an electroplating solution have similar reduction potentials, the weight ratio of the metals deposited as an alloy tend to be similar to the ratio of concentrations of the metal ions in the electroplating solution. This characteristic lends itself to predictability and control of the deposited alloy composition within the deposited feature. In contrast, when the alloying metals do not have similar reduction potentials, prediction and control of the alloy composition within the deposited feature becomes more challenging.
Lead-free solders and Permalloy are formed from alloying metals that have substantially different reduction potentials. In addition, the baths used to electroplate lead-free solders and Permalloy include the alloying metals in much different compositions. For example, for tin-silver solders, the silver concentration in the electroplating bath is much less than the tin concentration. In electroplating baths from which Permalloys are deposited, the non-nickel metal ion concentration is generally much less than the concentration of nickel ions. These factors can contribute to variations in the ratio of metals at different locations within the deposited alloys. Such variations can be undesirable for several reasons.
Near-eutectic tin alloy based solders are desirable due to properties such as good melting, solderability, ability to cope with lead contamination and reliability. To use these alloys as general purpose solders, the silver and copper contents are limited to near eutectic compositions on account of melting point limitations for bumping applications. If the weight ratio of metals at different locations within the formed feature varies too far from the ratio needed to provide an alloy with the desired melting properties, incomplete reflow may result at those locations and some of the solder alloy may not melt at the reflow temperature. Simply elevating the reflow temperature is often not a solution because higher temperatures may damage surrounding architecture. Incomplete reflow is undesirable because it can adversely affect conductivity properties of the alloy and integrity of intermetallic interfaces.
In magnetic head applications, variations in magnetic properties of a magnetic head are undesirable because of their impact on performance of the head. The magnetic properties of Permalloy are affected by the composition of the alloy. If portions of the deposited Permalloy differ in composition from other portions, performance of the magnetic head may be adversely affected.
In some applications it may be desirable to produce metal alloy features where portions of the alloy feature differ in composition from other portions of the alloy feature. For example, when an alloy feature interfaces with two dissimilar materials, it may be desirable for a portion of the alloy feature to have one composition at the interface with one material and a different composition at the interface with another material.